Fix error messages in folly/dynamic

Summary:
Some of the error messages had the incorrect expected types.
Fix those occurences

Reviewed By: shixiao

Differential Revision: D10003775

fbshipit-source-id: ba8e3b7d14ee2c5442dfd24b9fbf7775d601f6f0

folly/dynamic.cpp

@@ -190,7 +190,7 @@ dynamic const& dynamic::atImpl(dynamic const& idx) const& {
 dynamic const& dynamic::at(StringPiece idx) const& {
   auto* pobject = get_nothrow<ObjectImpl>();
   if (!pobject) {
-    throw_exception<TypeError>("object/array", type());
+    throw_exception<TypeError>("object", type());
   }
   auto it = pobject->find(idx);
   if (it == pobject->end()) {
@@ -270,7 +270,7 @@ std::size_t dynamic::size() const {
   if (auto* str = get_nothrow<std::string>()) {
     return str->size();
   }
-  throw_exception<TypeError>("array/object", type());
+  throw_exception<TypeError>("array/object/string", type());
 }
 
 dynamic::iterator dynamic::erase(const_iterator first, const_iterator last) {